tabbox: Show window switcher only when there are two or more windows

This merge request addresses two confirmed bugs

• https://bugs.kde.org/show_bug.cgi?id=370396
• https://bugs.kde.org/show_bug.cgi?id=419408

To summarize, when the Include "Show Desktop" icon option is disabled, users expect the switcher to not show the Desktop "window" in the switcher.

For reference, the Include "Show Desktop" icon option is seen here:

Examples of the switcher showing the Desktop "window"

1. Switching between All Windows, when no windows are open

   switch-windows-show-desktop-disabled-no-windows

2. Switching between All Windows, when one window is open

   switch-windows-show-desktop-disabled-one-window

3. Switching between Current application windows, when no windows is open:

   switch-app-windows-show-desktop-disabled-no-windows

4. Switching between Current application windows, when one window is open:

   switch-app-windows-show-desktop-disabled-one-window

With this merge request, the switcher does not show the Desktop "window" if the Include "Show Desktop" icon is disabled. So, for each of the above examples, the switcher would not appear to the user.